Water as a Base: Water (H₂O) acts as a Bronsted-Lowry base in an acid-base reaction.
Generic Acid (HA): Transfers a proton (H⁺) to water.
Reaction Steps
Proton Transfer: Lone pair of electrons on water's oxygen atom pick up a proton from HA, making water H₃O⁺ (hydronium), and HA becomes A⁻ (conjugate base).
Charge and Electrons:
Water becomes H₃O⁺ with a positive charge.
HA loses a proton, becoming A⁻ with a negative charge.
Bronsted-Lowry Acid-Base Concepts
HA: Bronsted-Lowry acid (donates a proton).
A⁻: Conjugate base of HA.
H₂O: Bronsted-Lowry base (accepts a proton).
H₃O⁺: Conjugate acid of water.
Equilibrium and Ionization Constants
Equilibrium Expression: Products over reactants.
Exclude pure liquids like water.
Ka (Acid Ionization Constant):
Indicates the strength of an acid.
Calculated as [H₃O⁺] x [A⁻] / [HA].
Strong Acids
Example: HCl
Functions as a Bronsted-Lowry acid by donating a proton to water.
Almost 100% ionization; equilibrium heavily favors products.
Ka: Very large (>1), indicating strong acid.
Conjugate Base (Cl⁻): Weaker due to strong acid.
Weak Acids
Example: Acetic Acid (CH₃COOH)
Acts as a Bronsted-Lowry acid.
Weak acid: Does not donate protons efficiently.
Ka: Much less than 1, indicating weak acid.
Equilibrium Position:
Lies to the left (more reactants than products).
General Concepts
Stronger Acid: Weaker conjugate base.
Competing Base Strength:
Water as a Bronsted-Lowry base versus chloride anion.
Application
Acid Dissociation: Understanding strength and behavior using Ka values.
Strong acids: High Ka, almost complete ionization.